A throttle apparatus of dual cable/electronic control type, and a throttle apparatus of single electronic control type have been known as conventional throttle apparatuses applied to engines installed on four-wheeled vehicles.
For example, on an intake system provided with two surge tanks which are used to combine each three intake passages corresponding to respective cylinders on a V-type six-cylinder engine, and intake passages extending upstream from the respective surge tanks, the conventional dual-cable/electronic-controlled throttle apparatus interlocks two throttle valves with each other, which are disposed in the respective upstream intake passages, by means of a single throttle shaft, thereby driving the throttle valves to be opened/closed by means of a cable or a motor, and provides a return spring around the throttle shaft to return the throttle valves to a rest position on the close side (refer to patent document 1, for example).
The conventional electronic-controlled throttle apparatus rotatably combines throttle valves, which are disposed respectively in two intake passages formed on throttle body, by means of a single throttle shaft, thereby driving the throttle valves to be opened/closed by means of a motor disposed on one end of the throttle shaft, and returns the throttle valves to a rest position on the close side by means of a return spring disposed on the other end of the throttle shaft (refer to patent document 2, for example).
[Patent document 1]
Japanese Laid-Open Patent Publication (Kokai) No. H6-207535
[Patent document 2]
Japanese Laid-Open Patent Publication (Kokai) No. H8-218904
If the throttle shaft, namely the throttle valves are rotated toward the fully open side by means of the cable or the driving force of the motor on the above-mentioned apparatuses, the energizing force of the return spring increases from an initial set value (initial set force) Fo to the maximum value Fmax in proportional to the increase of the rotation angle θ as shown in FIG. 6.
On the other hand, as the energizing force required to return the throttle valves to the rest position on the close side, in consideration of the slide resistance of the cable or the rotation resistance and the like of the motor, although the initial set value Fo+α (<Fmax) is enough, the energizing force reaches the maximum value Fmax due to the characteristic of being proportional to the deformation quantity.
As a result, the load applied on the cable or the motor increases, and the operability degrades upon the drive by the cable, or the power consumption increases, or the size of the motor, consequently the size of the entire apparatus increases upon the drive by the motor.
The present invention is devised in view of the problems of the above-mentioned prior art, and has an object of providing a throttle apparatus which, for a configuration provided with a return spring that returns throttle valves, which are provided in intake passages, and carry out open/close operations, to a rest position, secures smooth open/close operations and a return operation while the maximum energizing force of a return spring is reduced, improves the operability, reduces the power consumption especially upon the open/close drive by means of a motor, reduces the size of the apparatus, and provides other advantages.